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LASEK vs. PRK: Comparison of Visual Outcomes

Minh Hanh Duong, MD and Damien Gatinel, MD

Service d’ophtalmologie, (Pr Hoang-Xuan), Hôpital Bichat, Fondation Rothschild,

Université Paris VII

Paris, France

Laser subepithelial keratomileusis (LASEK) combines the advantages of photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) (1,2). Corneal haze seems reduced in LASEK compared to PRK. Several factors that could explain the reduction of the intensity of the corneal haze in LASEK technique include corneal epithelium replacement, alcohol epithelial debridement, and corneal stromal patching.

This chapter reports the result of an investigation of the role of corneal epithelial replacement on postoperative pain, immediate visual recovery, and corneal haze by a comparative single masked study using the technique of LASEK or PRK with alcohol epithelial debridement. Myopic patients whose spherical equivalent was less than –5 diopters (D), and whose cylinder was less than or equal to 2 D were included in the study.

TECHNIQUE

In LASEK, epithelial debridement was initiated using an 8-mm blade of Hanna’s trephine to trephine corneal epithelium. Twenty percent diluted ethanol in salt balanced solution was placed inside the blade for 30 seconds. We were careful to avoid spillage on the untreated area. The ethanol was absorbed by a merocel sponge afterwards. The epithelium was then irrigated with a balanced salt solution. Creation of the epithelial flap began at the edge of the epithelial trephination using a Troutman forceps. The epithelium was raised as a flap, leaving a hinge in the cornea at the 3 or 9 o’clock position. During the creation of the epithelial flap, we had the option of irrigating the cornea to avoid corneal desiccation. After drying the surface with a merocel sponge, the laser was applied on the exposed bed. We used a Nidek EC 5000 excimer laser as the delivery system. Our nomogram was 10% under correction to the spherical equivalent of the intended treatment. The stroma was rinsed and the epithelial flap was replaced with the same Troutman forceps. At the end of the procedure, soft bandage contact lens was placed to secure the flap and topical antibiotic was applied.

In PRK, epithelial debridement and photorefractive keratectomy were performed similar to the LASEK technique. At the end of the procedure, the epithelial flap was removed, a soft bandage contact lens was placed, and topical antibiotic was applied.

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The patients were masked in terms of what procedure they underwent. However, our study design determined the first eye undergo LASEK and the second eye to undergo PRK. If the creation of the epithelial flap was unsuccessful in the first eye we converted to a PRK and treated the second eye with LASEK.

Postoperative treatment included antibiotic drops (ciprofloxacin) four times daily, and a combination steroidal anti-inflammatory and antibiotic drops (dexamethasone and neomycin) twice daily for the first 4 days. The soft contact lens bandage was removed at the fifth postoperative day. The patient received the combined steroidal antiinflammatory and antibiotic drops four times daily for the next 7 days.

To relieve postoperative pain, the patients were prescribed a combination of paracetamol (2,400 mg per day) and dextropopoxyphène (180 mg per day). The patients were examined at the first, fifth, and fifteenth postoperative day and then monthly thereafter.

We analysed maximal postoperative pain, refractive outcomes, and corneal haze. Maximal pain was graded according to an analogical visual scale from 0 to 10. At each examination, we measured manifest refraction and corneal haze levels by slit-lamp examination and graded the haze according to the Food and Drug Administration (FDA) guideline protocol. Analysis of visual acuity was performed after converting decimal visual acuity to LogMAR equivalent. Statistical analyses were performed with the paired Student t test.

RESULTS

We included 26 eyes of 13 patients, eight females and five males. Patients’ mean age was 30.4±5 (range 22–41) years. In the LASEK group, preoperative mean spherical equivalent was −2.6±1.1 diopters (D) (range −1.25 to −4.75), whereas in the PRK group it was −2.9±1.1 D (range −1.25 to −4.62). There was no statistical difference between the two groups. Mean preoperative astigmatism was 0.5±0.7 (range 0–2) D and 0.5±0.6 (range 0–1.75) in the LASEK and the PRK groups, respectively (non-statistically significant). Epithelial flap creation was unsuccessful in three cases (18.7%). Time spent to create the epithelial flap was approximately 8 minutes. We excluded one patient from our follow-up because the bandage contact lens and epithelium were removed on the first postoperative day.

Mean postoperative pain grading was available in 11 patients. It was 5.7±2.37 (range 1–9) and 5.3±2.3 (range 1–8) in the LASEK group and the PRK group, respectively. In one case in which the postoperative pain was not graded, the patient reported much more pain in the PRK operated eye than in the LASEK operated eye. There was no statistical difference in mean postoperative pain between the two groups. The mean difference in pain level between PRK eye and LASEK eye for each patient was 0.8± 1.5 (range −1 to 3).

Mean visual acuity recovery at the first postoperative examination was 0.63 ±0.16 (range 0.4–1.2) in the LASEK group and 0.70±0.36 (range 0.1–1.2) in the PRK group. There was no statistical significant difference between the two groups in terms of immediate visual recovery.

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Mean follow-up was 73.5±30.5 days (range 30–107) in the LASEK group and 73.9±46.6 days (range 71–197) in the PRK group. There was no statistical difference in follow-up duration between the two groups.

Mean postoperative spherical equivalent was 0.33±0.68 D (range −1 to +1.5) in the LASEK group and 0.36±0.75 D (range −1.25 to +1.5). There was no statistical difference between the two groups concerning spherical equivalent outcomes.

Mean postoperative astigmatism was 0.21±0.51 D (range 0–1.75) in the LASEK group and 0.23±0.36 D (range 0–1) in the PRK group. There was no statistical significant difference in postoperative astigmatism between the two groups.

Best-corrected visual acuity (BCVA) postoperatively was 1.0±0.10 (range 0.7–1.2) in the LASEK group and 1.1±0.06 (range 0.8–1.2) in the PRK group. In the LASEK group, one patient lost 0.1 of BCVA, in the PRK group 2 patients lost 0.1 and 0.2 of BCVA. There was no statistical significant difference in postoperative BCVA between the two groups.

Mean postoperative corneal haze at the last examination was 0.5±0.3 (range 0–1) in the LASEK group and 0.5±0.5 (range 0–2) in the PRK group. There was no statistical significant difference in mean postoperative corneal haze between the two groups.

DISCUSSION

Mechanical epithelial debridement in traditional PRK for correction of low to moderate myopia induces moderate to severe pain, corneal haze, and delayed visual acuity recovery. By using the corneal epithelium to cover stroma after photoablation, the LASEK technique could theoretically maintain the safety of PRK while reducing pain, allowing for rapid corneal epithelium healing, and minimizing corneal haze.

The role of the epithelium in the healing mechanism is unclear. Other factors in LASEK technique, including the use of alcohol debridement or soft contact lens patch, could explain pain relief and minimal corneal haze.

In our single masked comparative study, we did not find any significant difference between LASEK and PRK with alcohol epithelial debridement concerning postoperative pain, refractive outcomes, and corneal haze. Corneal epithelial replacement in our study does not seem to play a role in postoperative pain and corneal haze intensity. In our study, both LASEK and PRK with alcohol epithelial debridement and soft contact lens patch induced little haze.

There were three other studies (3–5) comparing the effectiveness and safety of LASEK with those of PRK. Lee et al. and Autrata et al. showed that LASEK reduced the incidence of significant postoperative pain and corneal haze, while Litwak et al. (6) reported that PRK resulted in better outcome. In both studies, the surgeons used mechanical scraping as a method for de-epithelialization in PRK.

Lee et al. (3) performed a prospective study of 27 patients (54 eyes) with a manifest refraction of −3.00 to −6.50 D. PRK was performed in one eye and LASEK in the fellow eye. The choice of first eye to be treated and the surgical method were randomized. There was a 2-week interval between the procedures in all patients. Postoperative pain, epithelial healing time, uncorrected visual acuity (UCVA), manifest refraction, corneal haze, and surgical preference were examined. The patients were followed-up for 3

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months postoperatively. They reported that there were no significant between-eye differences in UCVA, refractive error, or epithelium healing time. However, LASEKtreated eyes had significantly lower postoperative pain scores and corneal haze than PRK-treated eyes. Further-more, at 3-month visit, 63% of the patients preferred the LASEK procedure.

Litwak et al. (6) also performed a prospective study on 50 eyes of 25 patients with mild to moderate myopia (mean preoperative spherical equivalent [MSE]) −3.00, range −0.75 to −7.75). At 1 month, the UCVA was similar in both groups. No eye lost lines of BCVA or developed haze. At 1 day, 72% of the patients reported more discomfort in the LASEK-treated eye compared to 24% who reported more about the PRK-treated eye. At 3 days, 80% reported problems with the LASEK-treated eye and 4% reported problems with the PRK eye.

More recently, Autrata et al. (5) reported results of a single-blinded, prospective, and comparative study between LASEK and PRK performed in 184 eyes of 92 patients with low to moderate myopia (MSE −4.65 D, range −1.75 to −7.50 D. At 1 week, the mean UCVA of the LASEK group was significantly better than the PRK group (0.64± 0.21 vs. 0.87±0.23, p=0.34). The difference in UCVA was not statistically significant after the 1- month visit. The mean pain level was significantly lower on days 1 and 3 in the LASEK eyes (p<0.05). The mean corneal haze level was lower in the LASEK group (0.21) compared to the PRK group (0.43) −p<0.05). Eighty six percent of the patients preferred LASEK to PRK.

There were several differences between these studies that may explain their different conclusions regarding postoperative discomfort. In Litwak et al. (6) the procedures were performed simultaneously in both eyes, whereas in Lee et al. (3) and Autrata et al. (5), there was a 2-week interval between procedures. The patients in these two studies were asked about discomfort immediately after the surgery and after 1 week, respectively. Furthermore, Litwak et al. found that it takes longer to loosen the corneal epithelium using diluted alcohol in Hispanic patients (minimal time to create a complete epithelial flap was 40 seconds).

Abad et al. (7,8) found that chemical de-epithelialization using 18% to 20% alcohol applied for 30 seconds could be associated with a quicker visual rehabilitation as compared to mechanical scraping. The authors also noted a trend toward less subjective haze in patients who had alcohol debridement. Stein et al. (9) used 25% diluted alcohol and reported similar results. Carones et al. (10) also found that using alcohol epithelial debridement is associated with less haze compared to mechanical epithelial debridement.

During wound healing after excimer laser photoablation, the keratocytes could be influenced by the regenerating epithelium (11). Covering the denuded surface of the cornea after PRK with the corneal epithelial flap may modify responses of the stromal keratocytes and the production of extracellular matrix and collagen, thereby reducing haze formation. In our study, we did not notice any difference in corneal haze with or without the epithelial flap. In a study with a follow-up of more than 1 year, Shah et al. (12) noticed that the haze was reduced when protecting the photoablated stroma with an epithelial flap without a soft bandage contact lens. We postulate that the second factor in corneal haze reduction after alcohol de-epithelialization could be the protection of the denuded stroma either by a soft bandage contact lens or by an epithelial flap. Further histological and large double-masked studies are necessary to confirm this hypothesis.

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ACKNOWLEDGMENT

The authors acknowledge the assistance of Dr. Puwat Charukamnoetkanok in the preparation and review of the manuscript.

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